1. Field of the Invention
The invention relates to a liquid crystal orientation layer and to a liquid crystal display element. In particular, the invention relates to a liquid crystal orientation layer and to a liquid crystal display element, each of which is used in a liquid crystal projector.
2. Description of the Related Art
In a liquid crystal display element, a TFT array substrate on a surface of which is formed an orientation layer for the purpose of orienting a liquid crystal and a counter substrate on which a similar orientation layer is formed are arranged opposite to each other, and a liquid crystal is filled between the two sheets of substrates.
As a method for imparting an orienting function to the foregoing liquid crystal orientation layer, a rubbing method of rubbing the liquid crystal orientation layer by a cloth and stretching the layer is chiefly employed. As an organic high molecular weight compound (polymer) for configuring a liquid crystal orientation layer, polyimides, polyamic acids, and so on are known. With respect to the molecular structure of polyimides or polyamic acids, from the viewpoint of reactivity (basicity) of the diamine, the major part of usual compounds generally has an aniline structure in which an aromatic ring (for example, a phenyl group) is bound to the diamine.
This phenyl group has an absorption band in an ultraviolet region at a wavelength of about 250 nm. For that reason, when a liquid crystal display element is irradiated with light over a long period of time, the polymer in the orientation layer is decomposed, thereby causing non-uniform display unevenness. This phenomenon becomes remarkable in the case of a liquid crystal projector using a light source capable of emitting light rays with high intensity or a liquid crystal display device using a high-luminance lamp. Among members configuring the liquid crystal display element, the orientation layer is mostly fast deteriorated. In order to improve the life of the liquid crystal display element, it is the most effective to improve the light fastness of the orientation layer.
Taking into consideration the foregoing circumstances, there are proposed liquid crystal orientation layers containing a polyamic acid or a polyimide with a reduced concentration of a phenyl group (see Patent Documents 1 and 2). However, since all of these proposed liquid crystal orientation layers are concerned with polymerization using a linear or alicyclic aliphatic diamine, the synthesis of a polymer having an average molecular weight of 100,000 or more as utilities for liquid crystal orientation is not simple, but a polymer having a relatively low molecular weight is obtained. Furthermore, since this polymer is a polymer having a large quantity of bindings leading to a low Tg from the structural standpoint, when formed into a film, the film becomes a liquid crystal orientation layer with low layer strength. When the layer strength of the liquid crystal orientation layer is low, scars or the like are generated due to rubbing at the time of rubbing treatment to cause display failure.
For the purpose of improving the adhesion and layer strength of the orientation layer, there is proposed a liquid crystal orienting agent resulting from mixing an epoxy compound represented by a prescribed chemical formula with a polymer selected among polyamic acids and polyimides (see Patent Document 3). As the epoxy compound described in Patent Document 3, there are proposed epoxy compounds including tertiary amine-containing epoxy compounds and aromatic ring-containing epoxy compounds. However, such an epoxy compound causes a lowering of the light fastness.
Patent Document 1: JP-A-2000-250047
Patent Document 2: JP-A-2004-170718
Patent Document 3: Japanese Patent No. 3206401